In industrial measuring and automation technology, especially in the context of the automation of chemical or process engineering sequences and/or the automatic control of industrial facilities, measuring units that are directly installed into the relevant facility are used, sometimes also referred to as field devices or field measuring devices, such as e.g. Coriolis' mass flow meters, density meters, magnetic inductive flow meters, vortex flow meters, ultrasound flow meters, thermal mass flow meters, pressure gauges, level meters, temperature meters, pH value meters etc, which each serve to determine measured values representing a time-dependent physical and/or chemical measurand, as well as the generation of at least one—digital or analogue measure value signal transferring such measured value outside the respective measuring device. Each measurand to be captured by the respective measuring device may, depending on the application, for example be a mass flow, a density, viscosity, a fill or limit level, pressure, pH value, electrical conductivity or a temperature or similar of a liquid, powdered, vaporous or gaseous medium that is transported or kept in an appropriate container, such as a pipe conduit or a tank. Measuring devices of this type, generally known to experts in the field, are demonstrated e.g. in European Patent EP A 1 591 977, British Patent GB A 22 29 897, US A 2001/0016802, US A 2010/0026322, US A 2011/0062942, US A 56 72 975, US A 60 14 100, US A 61 40 940, US B 64 52 493, US B 64 72 884, US B 66 84 340, US B 71 62 651, US B 72 96 482, US B 76 30 844, US B 77 78 784, US B 77 92 646, Published International Applications, WO A 00/26739, WO A 00/48157, WO A 01/71291, WO A 03/106931, WO A 2008/091548, WO A 2009/002341, WO A 2011/005938, WO A 2012/009003, WO A 2012/159683, WO A 88/02476, WO A 88/02853, WO A 94/20940, WO A 95/08123, and WO A 95/08758, or the applicant's own, not pre-published international application PCT/EP2012/057791, or are commercially offered by applicant's assignee itself, e.g. with the name t trend ATT12, Promag 53H, Prowirl 73F, Promass E 200, Promass F 200, Promass 83X, or Promass 84F.
Measuring units of the type described above are provided with an appropriate physical-electrical or chemical-electrical sensor to capture the pertinent process parameters. The sensor is usually inserted into the walls of the container for the medium, or into the conduit containing the medium, e.g. a pipe and serves to generate at least one initially analogue electrical measuring signal that corresponds to the measurand to be obtained, i.e. representing its chronological course; with said signal being further processed using a measuring device's electronics unit that is electrically connected to the transducer in such a way that relevant measured values for the measurand are obtained. The measuring device electronics unit of the respective measuring device is usually housed in a comparatively robust electronic housing protecting against shock, pressure, explosion and/or weather conditions. This housing may be placed away from the sensor and only linked to the latter via a flexible line; or it may be directly placed at the sensor or in a separate sensor housing for the sensor. The measuring device electronics unit for measuring units, such as the ones discussed here, is further during operation electrically connected via appropriate connection terminals and linked electrical connection cables with a higher-level electronic data processing system that is usually distanced in space from the relevant measuring device. Such data processing system receives the measured values generated by the respective measuring device through the measure value signal in a format it can process. In metrology, not least for the case of a transfer of measure value signals over larger distances, e.g. in a range of 10 m to a couple of hundred meters, the use of analogue electrical current signals, i.e. analogue measure value signals, for which a momentary amperage of a pre-set, but within a set measuring range, namely within a current range reserved for the transfer of measured values, variable signal current each represents exactly one measured value for the measurand. In industrial metrology, the transfer of measured values is often effected using so-called 4-20 mA current loops, therefore, such signal currents are used as measure value signals, which are variable within a lower limiting current strength set at approx. 3.8 mA—occasionally also referred to as live zero point or live zero value—and an upper limiting current strength set at approx. 20.5 mA. Amperage ranges below and above said measuring range, also e.g. defined in the DIN IEC 60381-1 standard, for 4-20 mA current loops are usually reserved for signaling previously defined special operation conditions deviating from the normal measuring operation in the normal operation mode corresponding to the measuring unit, e.g. special operating mode alarm conditions due to a measurand outside the measuring range specified for the measuring device, or due to the failure of the respective sensor, also to comply with the requirements set out in the NAMUR recommendations NE43:18.01.1994 for unified signal levels for the failure information of digital transducers with an analogue output signal.
In order to generate the measure value signal, modern measuring devices of the type discussed here first allow the generation of a digital measuring signal representing the analogue electrical measuring signal of the transducer using an analogue to digital transformer placed e.g. directly at the sensor and/or inside the electronics housing mentioned above. Further processing of the digital measuring signal for the purpose of generating digital measured values representing the relevant measurand, and for the conversion of said measured values in at least one measure value signal of the type as described above externally to the measuring system electronics, requires the measuring device electronics to include a converter circuit receiving the digital measuring signal. As, for example, also shown in the respective US A 2001/0016802, US A 2010/0026322, US A 2011/0062942, US B 76 30 844, US B 77 92 646, US B 77 78 784, US B 64 52 493, and US A 60 14 100, Publication International Application WO A 95/08123 or WO A /009003 referred to initially, said converter circuits in modern measuring devices of the type discussed here usually consist of a digital micro-processor containing one, occasionally more than one processor and/or a digital signal processor (DSP); with said micro-processor receiving the digital measuring signal via a measuring signal input. The micro-processors are, inter alia, arranged in such a way as to generate a sequence of measured values, i.e. sequence representing a chronological line of the measurand for digital measured values, each one representing a momentary measurand at different times.
For the case that the measure value signal is to be output as an analogue current signal, said converter circuit also shows a current interface controlled by the micro-processor with at least one current output and a control input that is designed to let the signal current flow through the current output while adjusting both its amperage to a stationary amperage level corresponding to the momentary control value at said control input as determined by the micro-processor for the appropriate control output in such a way that each of the stationary amperage level depends on a respective control value according to a characteristic curve function inherent to the current interface by the operation parameters of the electronic components constituting the current interface. The current interface may take the form of a passive interface, i.e. an interface inserting a current in the sense of a load modulation provided by a supply circuit outside of the measuring device, or an active interface, i.e. an interface varying a current provided by an internal supply circuit in the measuring device. Inside the micro-processor, there is furthermore a respective appropriate calculation rule determined by at least two pre-set coefficients which defines how to calculate each of the control values of the control value sequences depending on the digital measuring values of the measured value sequence. The calculation rule is usually a linear function or polynomial function of polynomial degree One determined by exactly two coefficients. On the whole, the converter circuits of the type discussed here thus have a characteristic converter curve determined by the said calculation rule and the characteristic curve function inherent in the current interface that transforms each of the measured values into a respective current amperage level, or according to which a measurand within each pre-set measuring range is projected onto the signal current.
Usually, such current interfaces are made using a linear current controller and an upstream digital-to-analogue transducer to convert the digital control value into a respective analogue one for the signal current to be adjusted. The linear current controller captures the momentary amperage level in such a setup via the falling analogue measuring voltage at a measuring resistor the signal current flows through. As further shown in WO A 95/08123 referred to above, the current interfaces may also include a current signal output by the analogue to digital converter that digitalizes said measuring voltage. At this is output a sequence of current values, i.e. a sequence of digital current values representing the respective amperage at different moments in time which, in turn, represent the chronological sequence of the actually set current strength of the signal current. Via a current signal input on the micro-processor appropriately connected to said current signal output, the digital current values can be read by the micro-processor and processed further, e.g. to determine a relationship between a terminal voltage measurable between two connected terminals each linked with a connecting lead outside the measuring unit carrying the signal current during operation, and the actually set amperage for the signal current via the current interface.
Measuring devices of the type discussed here must occasionally be checked after initial start-up, especially with regard to a calibration adjustment—be it on the instructions of the operator running the measuring device and/or on the order of the regulatory authority supervising the measuring center the measuring device is part of—to see whether the required measuring accuracy or the one stated in the specifications, i.e. that accuracy with which the measurand eventually is projected onto the measure value signal, is still reliably reached. The respective current interface also receives special attention when measuring devices of the type discussed here are checked. This is not least done because one regularly has to assume that the characteristic curve function currently assigned to the current interface that is the basis of the signal value amperage in relation to the appropriate control value at any moment may actually deviate from the characteristic curve function originally set at the respective current interface at an earlier time, e.g. during manufacturer calibration or during measuring device set-up due to ageing. The current interface is traditionally checked by looping a current measuring device, e.g. a digital ampere meter into the circuit made by the current interface in such a way that the respective measuring device may also carry the signal current, and the micro-processor then executes an appropriate testing program triggered by an appropriate start command which transfers a group of previously defined test control values to the current interface one by one and generates a corresponding sequence of amperage levels. Usually, also those control values are used as testing control values that correspond to the limit amperage of the lower range limit and/or the upper range limit, so for example correspond to control values for 4 mA or 20 mA respectively. The actually set stationary amperage level in relation to the current testing control value is precisely measured in each case using the current measuring device and then displayed accordingly. For documentation purposes of the test, the testing control values can also be saved together with the respective measuring values for actually set amperage levels also with a digital recording unit which may, for example, be directly connected to the current measuring device or implemented in it. Similarly, the testing control values respectively set by the micro-processor on the measuring device may be displayed and/or transferred to the digital recording unit for storage—which communicates with the micro-processor at least for this case using an appropriate service interface in the measuring device electronics.
The measuring values determined by the current measuring device for the individual amperage levels are then compared to the current readings with their respective testing control value in such a way as to determine any deviation of the actually set stationary amperage level to the respective testing control value. Should all deviations remain within a pre-set tolerance range, and thus no significant deviation of the current characteristic curve function to the original one at the current interface is determined, the check of the respective current interface may be regarded as passed, otherwise an appropriate realignment of the converter circuit may be required. Such a realignment may be completed for traditional converter circuits e.g. by reading the measuring values for the different amperage levels into the micro-processor, e.g. by manual input via a connected keyboard or by direct reading from the above-mentioned recording unit, and the micro-processor then calculates a new set of replacement coefficients for the calculation rule based on the respective measuring values, the corresponding testing control values and the coefficients saved in the micro-processor. As a consequence, the calculation rule is modified as the replacement coefficients are then used for the calculation rule in place of the original coefficients. The modification of said calculation rules may also be effected for traditional measuring devices by first reading the measured values for the amperage levels, the corresponding testing control values together with the coefficients saved in the micro-processor first using a portable computer, a laptop or tablet computer that is ready outside the measuring unit, if necessary via a service interface or radio interface, and then the respective replacement coefficients are calculated to then in turn be transferred to the micro-processor of the converter circuit.
One disadvantage of traditional measuring device electronics of the type discussed above, or the converter circuits implemented in them is that normal measuring operations of the measuring unit must be interrupted to check its current interface, i.e. the measuring site in the supervised facility part must be down for the duration of the control process. In addition, such checks require a usually very expensive current measuring unit which in turn also has to be calibrated regularly, i.e. a very special testing means. Another disadvantage in the traditional measuring device electronics of the type discussed above, or their respective converter circuits in this context is also that firstly checking the current interface requires considerable manual handling of the respective measuring system consisting of the measuring device subject to the check and the data processing system linked to it, in this case the temporary looping of the current measuring unit that is an essential element of the check, and that secondly the recurring modification of the calculation rule for the control values require other special testing means in the shape of a suitable communicating recording unit and/or a computer that is programmed accordingly, and thus considerable additional technical efforts.